1) Field of the Invention
The present invention relates to a Raman amplifying device including a plurality of Raman amplifiers having a gain characteristic with a gain peak at which an amplification gain is maximized.
2) Description of the Related Art
Development of optical communication systems that have a large transmission capacity and can perform long distance transmission is under way, with the recent development of optical communications such as the Internet. From the standpoint of increasing the transmission capacity, a wavelength division multiplexing (WDM) method and a dense wavelength division multiplexing (DWDM) method have been proposed and put into practical use. The WDM method is a communication method in which a plurality of signal lights having a different wavelength is transmitted in the same optical transmission path, and the DWDM method is a communication method in which more signal lights than in the WDM method are transmitted in the same optical transmission path. By adopting these communication methods, the quantity of signals that can be transmitted at the same time increases, thereby enabling an increase in the transmission capacity in the optical communications.
From the standpoint of realizing the long distance transmission, it is widely practiced to arrange optical amplifying devices that amplify the intensity of the signal light attenuated on the way of transmission through the optical communication path. As the optical amplifying device, an erbium doped fiber amplifier (EDFA) and a Raman amplifying device using Raman amplification are well known. Particularly, the Raman amplifying device can amplify a light of an optional wavelength by changing the wavelength of a pump light, thereby having an advantage in that there is a wide room for choice of the signal light.
When an optical communication system combining the WDM method or the DWDM method with the Raman amplification is to be realized, it is important to provide an amplification gain of the same level with respect to signal lights having transmitted through the same transmission path and having a different wavelength from each other. Particularly, when a plurality of Raman amplifying devices is arranged on the transmission path, since a gain deviation of the individual Raman amplifying device is accumulated, flattening of the gain wavelength characteristic of the optical amplifier is an important issue.
Therefore, an example is heretofore disclosed in which one Raman amplifier referred to as a W-type, having a peak (hereinafter, “gain peak”) of the gain wavelength characteristic in a wavelength band of from 1539 to 1579 nanometers, at the opposite ends and substantially at the center of the band, and one Raman amplifier referred to as an M-type, having a gain peak between the gain peaks of the W-type Raman amplifier are combined to form a Raman amplifying device (for example, see Optical Amplifiers and There Application 2001 July OTuA3).
However, flattening of the gain wavelength characteristic cannot be sufficiently realized as a whole, only by the configuration in which two Raman amplifiers are simply combined to simply compensate for peaks and valleys in the respective gain wavelength characteristics.
Furthermore, even when the Raman amplifying device is formed as in the conventional art, when the Raman amplifying device is actually assembled in the optical communication system, the gain wavelength characteristic may not become flat. That is, in the optical communication system in which long distance transmission is performed, a Raman gain coefficient in optical fibers for performing Raman amplification or an attenuation constant of an input pump light may change from the values at the time of designing, due to fluctuations in the temperature or the like in the external environment. Therefore, even if the Raman amplifying device has a flat gain wavelength characteristic at the time of designing, the gain wavelength characteristic may be changed when the optical communication system is actually laid down.
In the actual optical communication system, problems due to a malfunction in a pump light source or the like, which forms the Raman amplifying device, should be taken into consideration. In the optical communication system laid down in a wide range, determination of the spot having the malfunction is not easy, and repair work may be difficult according to the place of the spot having the malfunction. Therefore, it is desired to provide a mechanism that can maintain the gain deviation in the optical communication system in a certain range, even when a part of the optical communication system has a malfunction.
Furthermore, when a system in which the Raman amplifiers are connected in multiple stages is designed according to the conventional art, it is necessary to design the gain wavelength characteristic of the individual Raman amplifier separately, so that a desired gain wavelength characteristic can be obtained as the entire system. Therefore, there is a problem in that the designing process becomes very complicated and difficult.
The present invention has been achieved in order to solve the above problems in the conventional art, and it is an object of the present invention to realize a Raman amplifying device including a plurality of Raman amplifiers and capable of maintaining the gain deviation within a certain tolerance, and a Raman amplifying system combining a plurality of Raman amplifying devices.